Cozumel is a relatively flat island based on limestone, resulting in a karst topography. The highest natural point on the island is just 15 m (49 ft) above sea level. The cenotes are deep water filled sinkholes formed by water percolating through the soft limestone soil during thousands of years. Cozumel’s cenotes have very restricted access available only to well qualified cave divers with appropriate registration. In the early 1990s, a group of cave explorers here discovered the 5th largest underwater cave in the world.